1. Field of the Invention
This invention relates to integrated circuit (IC) wafers of the type including Zener diodes to provide reference voltages. More particularly, this invention relates to forming such a Zener diode wherein the junction breakdown occurs in a sub-surface region in order to insure stable operation.
2. Description of the Prior Art
U.S. Pat. No. 4,213,806, issued to W. K. Tsang on July 22, 1980, describes a method for diffusing wafers so as to form IC chips with Zener diodes having sub-surface breakdown junctions, i.e. so-called buried Zener diodes. The procedure employs a triple diffusion process compatible with conventional bipolar processing. In the first step, a deep p++ diffusion is applied, reaching through the epitaxial layer (Epi) to the buried n+ layer; this deep diffusion is carried out simultaneously with the isolation diffusion for the other components of the wafer. Next, a shallow p+ diffusion is formed over the deep p++ diffusion, and is arranged to extend out laterally well beyond opposite edges of the deep diffusion. This second diffusion is carried out simultaneously with the base diffusion for the wafer. Finally, a shallow n+ diffusion is applied over the p diffusions, to form a sub-surface breakdown junction with the p-doped material, adjacent the central regions thereof, and avoiding breakdown at the surface junction between the n+ and p+ diffusions. This last diffusion is carried out simultaneously with the emitter diffusion for the wafer.
The procedures described in the above Tsang patent have gone into substantial commercial use, and have provided excellent results. However, difficulties have arisen when attempts have been made to apply that procedure to relatively large-sized wafers such as those 4" in diameter. Specifically, it has been found that there is a tendency for the Zener diodes to break down at higher than specified voltages, approaching and often exceeding the break down at surface junctions thus causing potentially unstable performance.
Accordingly, there has developed a need for improved procedures for producing good quality Zener diodes in relatively large-sized wafers.